might be the valve body.
it could be gummed up if the car sat for a REALLY long time.
you would have to crack open the pan, drain the fluid, and unbolt the filter, and then about 10 more bolts drops the valve body. before you drop the valve body you could also visually check if the shifter is moving the parts on the valve body, you shine a light on the drivers side and have somebody move the shifter and watch the linkage. you could have a broken linkage that moves but doesn't move the selector on the valve body.
the valve body is a hydraulic circuit board, it has springs and little ball bearings that go in certain places to control the destination of the transmission fluid depending on gear. you have to be careful dropping it to make sure the reverse curcuit ball bearing doesn't fall out of the circuit.
on the sides of the valve body are the holders for the various springs. and valves, the circuit also splits in half and there is a gasket between them that you may want to replace.
Its not hard to do this but you want to make sure you remember where things go. a manual is good one specific to the C6. people also like to add shift kits when they do this. a shift kit makes the gear changes shorter and it snaps the car in gear 1,2,3. this makes the clutch packs last longer but the shifting is very abrupt and you may think the trans is malfunctioning.
I would look at the valve body first since the car stopped moving in 1995, high fluid might mean somebody thought the trans was not engaging due to low fluid so they just kept adding fluid hoping the trans would start to engage again. there is most likely a deposit inside the valve body clogging it up and preventing the fluid from engaging the gears. might also mean the clutch packs are worn and there was a lot of sediment inside the trans when it was parked, and then everything settled and turned to concrete. well you will need to get into the trans to start.
that is a typical valve body overview might not match yours exactly there were different revisions.
you can see the upper part with springs and plungers if that is gummed up the trans won't come out of neutral. the slider is part of that unit so when you shift from the inside of the car it moves 7c398 back and forth that gets you into drive.
about 7 years ago i had to take mine apart and degum it, took me about 2 days working slowly on the bench taking photos and marking things so i knew how to put it back together.
